Thank you Dave. I have an understanding of constexpr evaluation, and realise the compiler is free to do what it likes in all but test4... I suppose I'd really like to know if there is an actual limit/threshold in place. If test3 is changed to use 100 characters it does as I expect, any more than that e.g. 101 and it bails. I also compiled with -Rpass-analysis='.*' -mllvm -print-after-all, and it *seems* the bail is in Induction Variable Simplify / Scalar Evolution, but I assume the actual problem could be before that.
-- Chris On 25/09/2019 20:57, David Blaikie wrote: > constexpr is a red herring here - except in 4, where you've used the > constexpr keyword to create a constexpr context, in 1-3 these are just > normal function calls the compiler optimizes as it sees fit - and it > seems it saw fit to unroll and optimize to a constant cases 1 and 2, > but not case 3 (perhaps because it was too long/some other middle-end > optimization decided to bail out). > > I couldn't say for sure exactly which LLVM optimization bailed out > early, or whether LLVM is using the same general approach as GCC here. > > Adding/removing the constexpr keyword from count_x shouldn't affect > anything in cases 1-3 (in either Clang or GCC, really). But looks like > it makes a big difference to GCC - perhaps GCC tries to evaluate > constexpr in the frontend even when the language doesn't require it. > Sounds like a recipe for some problematic compile-time to me... but > don't know. > > - Dave > > On Wed, Sep 25, 2019 at 12:37 PM Christopher Williams via cfe-users > <cfe-users@lists.llvm.org <mailto:cfe-users@lists.llvm.org>> wrote: > > Given the code below, clang produces a constant value for test1, test2 > and test4. Why doesn't it for test3? > This is more of a curious query, than a request for help, but if > someone > does have the answer, I'd appreciate as much detail as possible. > || > > |staticconstexprintcount_x(constchar*str){intcount{};for(;*str > !=0;++str){count +=*str > > > =='x';}returncount;}#defineSTRx1"123456789x"#defineSTRx4STRx1STRx1STRx1STRx1#defineSTRx8STRx4STRx4#defineSTRx16STRx8STRx8inttest1(){returncount_x(STRx4);}inttest2(){returncount_x(STRx8);}inttest3(){returncount_x(STRx16);}inttest4(){constexprautok > =count_x(STRx16);returnk;}| > > > |test1():# @test1()mov eax,4ret test2():# @test2()mov eax,8ret > test3():# @test3()xor eax,eax mov dl,49mov ecx,offset > .L.str.2+1.LBB2_1:# =>This Inner Loop Header: Depth=1xor > esi,esi cmp > dl,120sete sil add eax,esi movzx edx,byte ptr [rcx]add rcx,1test > dl,dl jne .LBB2_1 ret test4():# @test4()mov eax,16ret > .L.str.2:.asciz > > > "123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x"| > > gcc does: > > |test1():mov eax,4ret test2():mov eax,8ret test3():mov eax,16ret > test4():mov eax,16ret| > > Compilation command lines used: > > |clang++-Ofast-std=c++2a-S -o --c src/test.cpp |grep -Ev$'^\t+\\.'gcc9 > -Ofast-std=c++2a-S -o --c src/test.cpp |grep -Ev$'^\t+\\.'| > > Compiler Explorer: https://godbolt.org/z/V-3MEp > > > > _______________________________________________ > cfe-users mailing list > cfe-users@lists.llvm.org <mailto:cfe-users@lists.llvm.org> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users >
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